Recently, the amounts of plastics, fibers, paper, etc. contained in urban refuse have been increasing due to diversification of physical distribution systems and the like and there has therefore been a growing tendency for urban refuse to become high-calorie refuse.
Plastics, which cause local high-temperature heat generation during the incineration process, damage the refractories or generate clinkers, thus obstructing the continuous operation of the furnace and the incineration of refuse at a rated capacity. Therefore, plastic may be sorted out as being a substance unsuitable for incineration. However, it is difficult to completely remove all of it from the refuse. Further, if such refuse is not incinerated but used for land reclamation, substances which would otherwise be used as valuable energy sources are dumped without being effectively utilized.
In addition, since the gas temperature at the furnace outlet is maintained at 700.degree. C. to 950.degree. C. to incinerate high-calorie refuse as it is, cooling must be effected with a large amount of air or spray water, so it is the present trend for the size of the furnaces to increase. The lower limit value (700.degree. C.) for the furnace outlet gas temperature is set principally with a view to preventing offensive odors. The upper limit value (950.degree. C.) is set from the viewpoint of the furnace operation so that dust which melts at high temperature will not adhere to the flue or the like downstream of the outlet of the combustion chamber so as to avoid trouble.
On the other hand, in November, 1983, harmful dioxins and the like were detected in mechanical furnaces of the stoker type, which gave rise to a social problem. It is considered that plastics are the main cause of generation of harmful organic chlorine compounds, including polychlorodibenzoparadioxins (hereinafter abbreviated as "PCDDs") and polychlorodibenzofurans (PCDFs) which are chemically very similar to the PCDDs. At present, generation of such organic chlorine compounds takes place irrespective of the type of furnaces, whether mechanical furnaces or fluidized bed furnaces.
It has heretofore been pointed out by many researchers that chlorobenzenes (CBs) and chlorophenols (CPs) are strongly related to generation of such harmful compounds, as being precursors thereof. It has been reported that the amount of generation of such harmful compounds is likely to increase as the exhaust gas flows from the furnace to the flue, that is, as the exhaust gas temperature decreases, and that these harmful compounds are generated by the electric discharge effect within the electric precipitator.
Although most of the mechanism of generation of PCDDs and the like in the furnaces has not yet been clarified, it is generally said that these compounds are generated at a temperature not higher than 700.degree. C. and are decomposed by oxidation treatment at 900.degree. C. to 1200.degree. C.
In the case of a mechanical furnace, the inside of the waste stacked on the stoker in the furnace bottom is in a baked state at 300.degree. C. to 400.degree. C.; therefore, this is a region where dioxins are likely to be generated. Moreover, since the air ratio for combustion in a mechanical furnace is as high as 2 or more, the rate at which the refuse is cooled by air is high, so that it is difficult to raise the temperature in the upper part of the furnace to 1200.degree. C. unless the calorific value of the object of combustion is high. If the calorific value is high, the temperature becomes locally high, which causes problems such as damage to the refractories and generation of clinkers.
In the case of a fluidized bed furnace, the furnace bottom is constituted of a fluidized bed formed from a fluidizing medium, for example, siliceous sand, and the furnace is usually operated at about 700.degree. C. to 900.degree. C. Therefore, it is more advantageous than the above-described mechanical furnace. However, the fluidized bed furnace has a complicated chemical reaction region in the fluidized bed, so that it cannot be said to be possible to reliably prevent generation of dioxins or the like even with the fluidized bed.
In general, fluidized bed furnaces that burn refuse use siliceous sand (SiO.sub.2) having an average particle diameter of about 0.4 to 2.0 mm as a fluidizing medium and maintain the temperature of the fluidized bed formed from the fluidizing medium at about 700.degree. C. to 900.degree. C. to burn the refuse fed into the fluidized bed and further return the heat generated by combustion to the fluidizing medium to effect incineration.
Incidentally, the siliceous sand that is used as a fluidizing medium reacts with the following alkali metal compounds in a high-temperature region to form, for example, sodium silicate (Na.sub.2 O.3SiO.sub.2) in the form of water-glass, which cannot be fluidized; therefore, the temperature of the fluidized bed is limited by each particular object of combustion: EQU 3SiO.sub.2 +Na.sub.2 CO.sub.3 .fwdarw.Na.sub.2 O.3SiO.sub.2 +CO.sub.2 EQU 3SiO.sub.2 +2NaOH.sub.3 .fwdarw.Na.sub.2 O.3SiO.sub.2 +H.sub.2 O EQU 3SiO.sub.2 +2Na.sub.2 HCO.sub.3 .fwdarw.Na.sub.2 O.3SiO.sub.2 +H.sub.2 O+2CO.sub.2
More specifically, in a case where the weight proportion of the Na component (hereinafter referred to as Na concentration), as being a representative of the alkali metal compounds, to the amount of fluidizing medium (SiO.sub.2) is not greater than about 0.5% (i.e., in the case of ordinary urban refuse), the fluidized bed temperature must be limited to 900.degree. C. at maximum. In the case of sludge, industrial waste or the like which has a high content of alkali metal compounds and in which the Na concentration in the fluidizing medium is about 1%, the fluidized bed temperature must be limited to about 750.degree. C., paying some attention to safety. It should be noted that in order to suppress the reaction of sand and Na.sub.2 CO.sub.3 or NaOH, a certain melt retarder, for example, kaolin, can be added as an additive to incinerate waste containing alkali metal compounds or the like in the fluidized bed furnace. However, it is known that there is still a limitation on the concentration of alkali metal compounds in the waste that can be supplied per unit of time with respect to the amount of fluidizing sand retained; that is, there is a limit beyond which fluidization stops even if a large amount of melt retarder is added.
In a test of an experimental fluidized bed furnace, the Na concentration in sand at the time when the fluidization of the object of combustion undesirably stopped due to alkali metal compounds at a fluidizing medium (SiO.sub.2) temperature of about 800.degree. C. was about 0.6 to 1.8%, although it was dependent on the kind of alkaline metal compounds and the melt retarder used. If the particle diameter of the melt retarder added is small, the retarder may scatter as soon as it is cast into the furnace, resulting in no effectiveness. Further, addition of a melt retarder involves disadvantages, for example, a rise in the cost and an increase in the load on the devices in the subsequent stages, and therefore cannot be an effective measure to cope with the problem due to the presence of alkali metal compounds.
It is known that, when a variety of alkali metal compounds coexist together, they constitute a eutectic point and therefore melt at a lower temperature than the melting point of each of them when present alone. This is an important matter which must be taken into account when the fluidized bed furnace is operated and controlled. Since it is practically impossible to regulate the amount of alkali metal compounds mixed in urban refuse, it is important to control the temperature of the fluidized bed.
Accordingly, it is preferable in the fluidized bed furnace also to maintain the fluidized bed temperature at a level not higher than 900.degree. C., maintain the temperature of the free board section in the upper part of the furnace at 900.degree. C. to 1200.degree. C. and treat organic chlorine compounds such as dioxins and precursors thereof at a high temperature by mixing them with a sufficient amount of oxygen.
In addition, the melting point of fly ash in furnaces for incinerating urban refuse is about 1200.degree. C.; therefore, if the free board section is heated to 1200.degree. C. or higher, the problem of molten fly ash adhering to the flue is likely to occur.
In view of the foregoing points, the present invention aims at eliminating the above-described problems arising when waste, for example, urban refuse, is incinerated with a combustion apparatus such as a fluidized bed furnace or the like, and at providing a high-temperature and high-efficiency combustion apparatus and a combustion control method thereof which does not generate harmful substances such as dioxins.